Conventionally, there has been available a moving image reading mode (see the CCD reading method (the horizontal & vertical pixel addition technique (corresponding to VGA moving images)); see Internet <URL:http://www.sony.co.jp/Products/SC-HP/imagingdevice/ccd/tecvga.html>) of reading out pixels from an image sensor at high speed by pixel adding/thinning operation, in addition to a frame reading mode of sequentially reading out all pixels from the image sensor for still image photography. FIG. 11 is a view for explaining a conventional method of reading out signals from a general-purpose CCD image sensor in the moving image photography mode.
Referring to FIG. 11, after pixels are exposed, two pixels of the same color are added at a time in a vertical shift register. For example, pixels corresponding to numbers V in the vertical direction which satisfy V=10n (where n is 0 or a positive integer) and V=10n+2 are added, and pixels which satisfy V=10n+5 and V=10n+7 are added. The remaining pixels are not read out. Image signals added by a vertical shift register are transferred to a horizontal shift register.
In the horizontal shift register, pixels of the same color corresponding to numbers H in the horizontal direction which satisfy H=4m (m is 0 or a positive integer) and H=4m+2 are added, and pixels of the same color which satisfy H=4m+1 and H=4m+3 are added. The resultant pixel data are output.
Such an image sensor outputs pixel data such that the number of pixels in the vertical direction is reduced to ⅕, and the number of pixels in the horizontal direction is reduced to ½.
In adding processing performed in the above manner, for example, as shown in FIG. 12, when G1[n+1] and G1[n+3] are added to output G1[K+1] and R[n] and R[n+2] are added to output R[K], a pixel coordinate inversion phenomenon occurs at G1[n+1] and R[n+2]. This results in the occurrence of jaggies (a stair-step pattern) on a fine object image.
More specifically, when pixels having undergone the above adding processing are thinned and reduced to be output as, for example, a VGA-size image, jaggies are made less noticeable. In contrast to this, as the size of an output image increases, the influence of jaggies increases. If, therefore, the size of an output image increases, a great deterioration in image quality may occur.
As described above, according to an addition reading method in a conventional image capturing apparatus, when, for example, a G1-R line is taken into consideration, the positional relationship between G1[n+1] and R[n+2] is reversed, as shown in FIG. 12. As a consequence, jaggies appear on an oblique line or the like, resulting in a great deterioration in image quality.
In general, in sampling an image, if the signal band of an original image is not reduced to a frequency (Nyquist frequency) ½ the sampling frequency, an aliasing signal is produced in a low-frequency signal. According to the addition reading method shown in FIGS. 11 and 12, pixel data are read out while the number of pixels in the horizontal direction is reduced to ½, and the number of pixels in the vertical direction is reduced to ⅕.
However, the above addition of pixels alone cannot sufficiently suppress the band of an original image. As a result, an aliasing signal is produced in a low-frequency signal.